It is commonly known to use light-curing inks when printing with inkjet printers. These contain multiple photoinitiators tuned to a predetermined short wavelength of light. Inkjet printers operable with light-curing inks comprise an illumination head for irradiation and illumination of the light-curing inks. During illumination, this head is moved along with the print head in traversing fashion between two housing sides of the inkjet printer, and irradiates the ink sprayed through print head nozzles onto the substrate. The wavelength of the emitted light is selected so that the photoinitiators are excited and the ink is at least partly polymerized. Polymerization results in an increase in the ink's viscosity, and in solidification of the ink. The degree of solidification depends on the irradiation duration and on the radiation power level of the illumination head.
The difficulty with inkjet printing, is that a very high radiation energy in a very short time is necessary in order to cure the ink sprayed onto the substrate. The energy necessary to cure UV-light-curing inks, is on the order of one joule. In order to apply the required energy even at high printing speeds, the radiation power levels required from an illumination head traveling along with the print head are in the kilowatt range. In the context of a fast-moving print head, an illumination head having a very high radiation power level must be selected so that the ink is solidified, and so that spreading of the ink is effectively prevented.
To achieve a sufficiently high radiation power level, the dimensions selected for the illumination head must be so large that it is no longer suitable for use in an inkjet printer embodied as a desktop unit. If a smaller illumination head is used in an inkjet printer embodied as a desktop unit, the risk then exists, especially when imprinting a non-absorbent substrate having capillary structures on the surface, that the ink applied onto the surface of the substrate will spread, and the printed image represented by the sprayed-on ink will become increasingly less sharp with time.
The spreading process depends on the ratio between the surface tensions of the ink and of the substrate, and thus also on the surface roughness of the substrate. The rougher the surface to be imprinted, the more quickly the ink spreads into the capillary structures. Although there is little ink spreading when the substrate has a smooth surface with no capillary structures, it is once again disadvantageous in this context that the adhesion between the ink and the surface of the substrate is low, and that a smear-proof bond thus cannot be achieved.
U.S. Pat. No. 6,092,980 describes a method for inkjet printing with light-curing ink that corresponds to the method cited above. In accordance therewith, a printing machine is used that comprises a carriage having a print head and an illumination head. After the printing of multiple line segments, a check is made as to whether the ink has cured. If that is not the case, provision can be made for additional passes over the printed image, during which only the illumination head, but not the print head, is active.
U.S. Patent Application Publication No. US 2003/0035037 further describes a method for inkjet printing with light-curing ink. Here the printing machine that is used comprises a carriage having a print head, on each side of which is arranged one illumination head. Only the illumination head that trails the print head along the relevant printed line is active in each case. The irradiation intensity of the two illumination heads is, however, not sufficient to cure the printed image line by line. A post-irradiation unit is therefore described, the substrate having the printed image being conveyed through beneath said unit.